The brief
The C180 had a coolant leak, the level dropping with a weep showing up around the engine. He brought it in, which is exactly right, driving with a coolant leak is hazardous and risks an overheat that can do real engine damage. When one of these leaks coolant from the engine, the coolant pump is a common source. The pump circulates coolant through the engine and the radiator so the heat goes somewhere, and on this engine the pump comes as a module with the thermostat. As the pump ages the seal weeps coolant and the bearing gets rough, and once it fails properly the engine can't shed heat and overheats. A weeping pump doesn't reseal, so it needs replacing as the module, with the thermostat that goes with it.
The diagnosis
A pressure test on the cooling system pinpointed it, the coolant pump module was weeping from its seal and losing pressure slowly, with the bearing starting to feel rough, which is the leak and the dropping level. The radiator, the hoses, the expansion tank and the rest of the system held fine. That's a pump module replacement, with the integrated thermostat and a fresh seal, rather than chasing a weep that's only going to fail harder.
The work
The cooling system was drained, the old coolant pump and thermostat module removed, and a new genuine Mercedes-spec module fitted with a fresh seal and the drive belt set back up properly. The system was refilled with the correct Mercedes coolant, the air bled out the proper way so no pockets were left, and held under pressure to confirm the seals were dry. A road test confirmed the gauge sat steady through traffic and at speed with no overheating and no noise, and the level stayed put.
The outcome
No more coolant loss, the level holding between checks, the gauge steady, the engine warming up on time, and no noise from the bay. The C180 went home with the cooling system circulating properly again. A weeping coolant pump only fails harder, and the failure at the end is an overheat that can take the head gasket with it, so changing the module kept it to a tidy job.
